Celestron 93507 manual Polar Alignment with optional Wedge, Finding the North Celestial Pole

Polar Alignment (with optional Wedge)

Even though the NexStar can precisely track a celestial object while in the Alt-Az position, it is still necessary to align the polar axis of the telescope (the fork arm) to the Earth's axis of

rotation in order to do long exposure astrophotography. To do an accurate polar alignment, the NexStar requires an optional equatorial wedge between the telescope and the tripod. This allows the telescope's tracking motors to rotate the telescope around the celestial pole, the same way as the stars. Without the equatorial wedge, you would notice the stars in the eyepiece would slowly rotate around the center of the field of view. Although this gradual rotation would go unnoticed when viewing with an eyepiece, it would be very noticeable on film.

Polar alignment is the process by which the telescope's axis of rotation (called the polar axis) is aligned (made parallel) with the Earth's axis of rotation. Once aligned, a telescope with a clock drive will track the stars as they move across the sky. The result is that objects observed through the telescope appear stationary (i.e., they will not drift out of the field of view). If not using the clock drive, all objects in the sky (day or night) will slowly drift out of the field. This motion is caused by the Earth's rotation.

Wedge Align

The NexStar has two equatorial wedge alignment modes (one for the northern hemisphere and one for the southern) that will help you polar align your telescope when using an optional equatorial wedge. After performing either an EQ AutoAlign or Two-Star Alignment, Wedge Align will slew the telescope to where Polaris should be. By using the tripod and wedge to center Polaris in the eyepiece, the fork arm (polar axis) will then be pointing towards the actual North Celestial Pole. Once Wedge Align is complete, you must re-align your telescope using either the EQ Two-Star or EQ AutoAlign methods. Follow these steps to Wedge Align the NexStar in the Northern Hemisphere:

Figure 6-3

This is how the telescope is to be set up for polar alignment. The tube should be parallel to the fork arm and the mount should point to Polaris.

1.With the telescope set up on an optional equatorial wedge and roughly positioned towards Polaris, align the NexStar using either the EQ AutoAlign or Two-Star Alignment method.

2.Select Wedge Align from the Utilities menu and press Enter.

Based on your current alignment, the NexStar will slew to where it thinks Polaris should be. Use the tripod and wedge adjustments to place Polaris in the center of the eyepiece. Do not use the direction buttons to position Polaris. Once Polaris is centered in the eyepiece press ENTER; the polar axis should then be

pointed towards the North Celestial Pole.

Finding the North Celestial Pole

In each hemisphere, there is a point in the sky around which all the other stars appear to rotate. These points are called the celestial poles and are named for the hemisphere in which they reside. For example, in the northern hemisphere all stars move around the north celestial pole. When the telescope's polar axis is pointed at the celestial pole, it is parallel to the Earth's rotational axis.

Many methods of polar alignment require that you know how to find the celestial pole by identifying stars in the area. For those in the northern hemisphere, finding the celestial pole is not too difficult. Fortunately, we have a naked eye star less than a degree away. This star, Polaris, is the end star in the handle of the Little Dipper. Since the Little Dipper (technically called Ursa Minor) is not one of the brightest constellations in the sky, it may be difficult to locate from urban areas. If this is the case, use the two end stars in the bowl of the Big Dipper (the pointer stars). Draw an imaginary line through them toward the

Figure 6-5

The position of the Big

Dipper changes throughout

the year and the night.

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